Ferromagnetic material

ABSTRACT

A ROOM TEMPERATURE STABLE FERROMGNETIC PERMANENT MAGNET MATERIAL WITH A LARGE FARADAY ROTATION AND HAVING A CURIE TEMPERATURE IN THE REGION OF SUBSTANTIALLY 180-220*C. OF A MUTUAL SOLID SOLUTION HAVING ATOMIC PROPORTIONS INDICATED BY THE FORMULA:   MN.8-1.0AL1.3-1.6GE.8-1.0O0.05-.3   IN THE FILM FORM, THE MAGNETIZATION IS NORMAL TO THE FILM PLANE.

United States Patent 3,795,541 Patented Mar. 5, 1974 ABSTRACT OF THEDISCLOSURE A room temperature stable ferromagnetic permanent magnetmaterial with a large Faraday rotation and having a Curie temperature inthe region of substantially 180-220 C. of a mutual solid solution havingatomic proportions indicated by the formula:

In the film form, the magnetization is normal to the film plane.

FIELD OF THE INVENTION This invention relates to ferromagnetic films ingeneral, and ferromagnetic films in particular of the general formulaMnAlGe, and in particular MnAlGe films of nonstoichiometric compositionand having low Curie point temperatures concurrent with high specificFaraday rotation.

PRIOR ART Ferromagnetic materials utilizing the constituents Mn-Al-Geare known in the prior art, as illustrated in US. Pat. No. 3,065,071,and discussed in the publication Manganese Aluminum Germanium Films forMagneto- Optic Applications, by R. C. Sherwood, E. A. Nesbit, I. H.Wernick, D. D. Bacon, A. I. Kurtzig and R. Wolfe, J. Appl. Phys., 42,1704 (1971). The above patent is directed toward compositions of thegeneral formula Mn Al Ge given in atomic proportions. It is noted in theabove patents that departures from the basic compositions disclosedresults in excessive deficiencies in magnetic properties when the limitsof the above formula are exceeded.

However, for various applications the properties sought cannot be metwithin the compositions known in the prior art. Thus, the prior artMnAlGe films disclosed while sufficient for permanent magnets'asdisclosed in the above patent, do not have the properties desired formany potential memory uses.

Potential memory use compositions preferably have room temperaturestability, and indeed should be stable over a wide temperature range inair. Further, these films should have a relatively low Curie point,which is also controllable as desired. Further, these films should havea high specific Faraday rotation, be easy to manufacture, and have asufficient coercivity. The remanent Faraday rotations should also beclose to that of saturation. Prior art films have not achieved this.

Thus, it is an object of this invention to provide a ferromagneticmaterial of a mutual solid solution structure that is stable at roomtemperature and to approximately 400 C.

A further object is to maintain the Curie point of this materialpreferably between 180 and 220 C., for use in potential memoryapplications.

Still another object of this invention is the ability to make an easilymanufactured film by a variety of techniques, the film being stable inair and having the above properties. Further, control of Curie pointsshould be easily obtainable.

Still another object is to maintain adequate magnetization andcoercivity of the film.

Still another object is a magnetic memory structure employing theferromagnetic compositions of this invention.

SUMMARY OF THE INVENTION These and other objects of the invention aremet by the solid solution consisting essentially of the followingmaterials having atomic proportions indicated by the formula:

Alternatively, oxygen may be added to the extent between 0.05-.3 inatomic proportion, for increased stability of the composition. Theproperties of these compositions and preferred embodiments are disclosedin the following general description.

GENERAL DESCRIPTION We have found a range of room temperature stablematerials having a large Faraday rotation with a magnetization normal tothe film plane and a Curie temperature between ISO-220 C. StochiometricMnAlGe, known in the art, has a high Curie temperature of approximately245 C. Stoichiometric thin films of this material have been prepared inthe prior art by DC sputter deposition and by vacuum evaporation.

We have prepared however, non-stoichiometric films of MnAlGe which allowgreater control and give the unexpected result of room temperaturestability, indeed, stability from room temperature to 400 C., and alower and controllable Curie point. These non-stochiometric films may beproduced by RF or DC sputtering, or from vacuum deposition techniques,all of these techniques known in the art. The substrate during suchdeposition techniques is essentially maintained in a temperature rangeof 25-420f C., while a deposition rate is maintained in the rangebetween 2-10 A. per second in a preferred embodiment. In this manner,films having a composition of Mn Al Ge and optionally, containing oxygen0.05-.3 are produced. These films have a Curie point between 180-220 C.,polar Faraday rotations at room temperature of between .75-.9 10deg./cm., a remanence of greater than .98 of saturation and coercivityof approximately 1500 oersteds. The saturation magnetization may bevaried between approximately 3000-3600 gauss.

While prior art disclosures of a magnetic composition of Mn Al Ge notedthat departures from the basic ternary composition result in excessdeficiencies in magnetic properties when the above is exceeded, we havefound that exceeding these limits results in an unusual and unexpectedresult of room temperature stability and Curie point control between 180and 220 C. while maintaining the fundamental magnetic properties of thematerial in a form suitable for magnetic storage devices.

Example I The composition Mn Al Ge in the form of a film varying from500-10,000 A., upon a nonmagnetic substrate, exhibited a Curie point of180 C., a specific Faraday rotation of .75 X 10 deg/cm. coercivity of1500 oersteds and a saturation magnetization near bulk, or approximately3000 gauss, the last three values measured at room temperature. TheFaraday rotation above and those in the following examples are forwavelengths in the visible and near infrared regions.

Example II The composition Mn Al Ge exhibited a Curie point of 220 C.with a specific Faraday rotation of .9 10 deg./cm., coercivity at 1500oersteds, and a saturation magnetization of approximately 3600 gauss.

Example III The addition of between 0.05 and .3 oxygen in atomiproportion, resulting in the formula was added to improve the chemicalstability of the material with no detrimental efifect of the magneticproperties as listed above for Mn Al Ge Example IV Example V As thealuminum amount increases, the Curie point decreases, but there is alimit to the non-magnetic additions as noted above as aifecting themagnetic properties and stability and crystal structure of thecompositions.

Example VI As manganese increases, so does it become necessary toincrease the germanium content, and this in turn increases the Curietemperature close to the bulk value.

Consequently, the ranges described above appear optimum for the purposesintended.

Thus, we have found it possible to control the Curie point of thesematerials between ISO-220 C., in a room temperature stable materialoptionally including oxygen as a constituent as desired. The Curiepoint, specific Faraday rotation, coercivity, saturation magnetizationand other propertiesare all desirable for storage applications. Themagnetization is normal to the film plane, as noted when grown by DC orRF sputtering or by vacuum deposition techniques.

Thus, what has been disclosed is a MnAlGe composition having a lowerCurie point between the range of 180- 220 C. that is stable between roomtemperature and substantially 400 C., can be manufactured by a varietyof techniques, is stable in air, and has the ease of composition controlallowing ease of Curie point control. Further, adequate magnetizationand coercivity is achievable for storage applications.

While these representations appear as molecular formmulas, thesubscripts should be considered to be atomic ratios and not asrepresenting an absolute number of atoms. Thus, the representations MnAl Ge designates a composition having the given atomic proportions ofeach constituent and not a molecule deficient in atoms.

These magnetic films are particularly useful for data storageapplications, such as in a beam addressable file wherein a polarizedbeam of light is directed toward the magnetic media and the degree ofrotation of the polarization of the beam from the area addressedindicates the state of magnetization of that area. The Faraday effect intransmission, or Kerr effect in reflection, may be used. Beamaddressable file configurations are known in the art. For; such a memoryapplication, a film of MnAlGe in the preferred proportions listed aboveis deposited upon a non-magnetic substrate. The substrate may betransparent as may be the film, for the particular wavelength used, orit may be opaque for reflection techniques. Thus, the substrate maytypically be of aluminum or other metals, of a ceramic, or glass. Filmthickness is between 50010,000 A. The film substrate structure may be indisk form, strip form, drum form or other forms known in the art. Filmdeposition is by methods known in the art and previously discussed.

Various other modifications as tothis composition and use are apparentto those skilled in the art.

What is claimed is:

1. A ferromagnetic composition having a Curie temperature betweensubstantially 180220 C. of a solid solution having atomic proportionsindicated by the formula:

3. A data storage medium comprising a magnetic film upon a non-magneticsubstrate, the magnetic film consisting essentially of a solid solutionhaving atomic proportions indicated by the formula:

References Cited UNITED STATES PATENTS 11/1962 Wernick -134 G 7/1972Bacon 75-134 G X OSCAR R. VERTIZ, Primary Examiner I. COOPER, AssistantExaminer US. Cl. X.R. 252-6251

